Now in Temporal Databases
Current date; Current time; Current timestamp; Until changed
The word now is a noun in the English language that means “at the present time.” This notion appears in databases in three guises. The first use of now is as a function within queries, views, assertions, etc. For instance, in SQL, CURRENT_DATE within queries, etc., returns the current date as an SQL DATE value; CURRENT_TIME and CURRENT_TIMESTAMP are also available. These constructs are nullary functions.
In the context of a transaction that contains more than one occurrence of these functions, the issue of which time value(s) to return when these functions are invoked becomes important. When having these functions return the same (or consistent) value, it becomes a challenge to select this time and to synchronize it with the serialization time of the transaction containing the query.
The second use is as a database variable used extensively in temporal data model proposals, primarily as timestamp values...
- 1.Ben-Zvi J. The time relational model. PhD Dissertation, University of California, Los Angeles; 1982.Google Scholar
- 2.Bliujūtė R, Jensen CS, Šaltenis S, Slivinskas G. Light-weight indexing of bitemporal data. In: Proceedings of the 12th International Conference on Scientific and Statistical Database Management; 2000. p. 125–38.Google Scholar
- 7.Lomet D, Snodgrass RT, Jensen CS. Exploiting the lock manager for timestamping. In: Proceedings of the International Conference on Database Engineering and Applications; 2005. p. 357–68.Google Scholar
- 9.Montague R. Formal philosophy: selected papers of Richard Montague. New Haven: Yale University Press; 1974.Google Scholar
- 14.Wiederhold G, Jajodia S, Litwin W. Integrating temporal data in a heterogeneous environment. In: Tansel A, Clifford J, Gadia SK, Jajodia S, Segev A, Snodgrass RT, editors. Temporal databases: theory, design, and implementation, Chapter 22. Redwood City: Benjamin/Cummings; 1993. p. 563–79.Google Scholar